WO2006000035A1

WO2006000035A1 - Construction material and powder coating composition

Info

Publication number: WO2006000035A1
Application number: PCT/AU2005/000919
Authority: WO
Inventors: David Murray James
Original assignee: Orica Australia Pty. Ltd.
Priority date: 2004-06-25
Filing date: 2005-06-24
Publication date: 2006-01-05
Also published as: NZ552374A

Abstract

A powder coating composition which comprises a polytetrafluoroethylene (PTFE) polymer. Also described is a powder coated fibre-cement soffit sheet and eaves liner board and a process for manufacturing said soffit sheet and boards.

Description

CONSTRUCTION MATERIAL AND POWDER COATING COMPOSITION

Field of the Invention This invention relates to cement-fibre liner boards for use under the eaves and soffit sheeting for use during the manufacture, renovation or repair of buildings such as residential housing.

Background of the Invention Residential housing is typically constructed with eaves, being the portion of the roof that extends beyond or overhangs the external sidewalls of the house. In this respect the soffit is the region under the eaves. Two construction styles are popular, open eaves, wherein the rafter tails, the portion of the rafters which overhang the wall, have eaves liner boards fastened thereto. These boards are attached to and across the topside of the rafter tails and conceal the roofing material located above. This style of construction leaves the rafter tails and the downward face of the liner boards visible and these surfaces are typically painted.

Traditional eaves liner boards are wooden planks which may have a rebated edge adapted to engage adjacent planks along its length. They can be readily nailed into place during the construction of the house. However, the boards and paint thereon will degrade over time due to the influence of water, vermin and heat and it can be difficult for a householder to repair damaged boards due to the height involved and since access to the boards typically requires the removal of the tiles or other roofing material located above the liner boards. The height of the eaves can also make painting, including the steps of cleaning, surface preparation and application of the paint, difficult for householders and a time consuming and expensive job for a tradesman.

The popular modern alternative construction style involves enclosing the eaves. Horizontal shafts (sometimes known as "soffits") extend from the wall to the end of the eaves, typically near the bottom of the fascia board. Soffit sheeting is fastened to the soffits to enclose the eaves and painted to protect it against the elements. The soffit sheeting used to be vinyl aluminium or steel sheets, but nowadays weather rated plywood may be used. A number of alternative materials have been used to a limited extent or have been suggested for use as soffit sheeting and include gypsum boards and fibre-cement boards. The fibre cement boards are composites formed from fibres such as asbestos or cellulose fibre with Portland cement as a binder.

Specialised fibre-cement products intended for use under eaves include Duralux™ sheeting provided by BGC (Australia) Pty. Ltd. This is a thin (4.5 mm) cellulose cement sheet with a bevelled edge intended for use under residential eaves and is available in Queensland, Australia. Other available cellulose-cement sheet products for use as soffit sheeting include Hardisoffit®, Silkline® and Villaboard®, all sold by James Hardie Aust. Pty. Ltd.

Cellulose fibre-cement sheeting are a replacement for asbestos fibre cement building sheets. Asbestos fibre cement composites were very widely used to produce roofing shingles and tiles, pipes and conduits and wall cladding. Corrugated sheeting was used as roofing and fencing material.

There are a number of problems with the use of cellulose fibre-cement products in external applications. Unlike asbestos sheeting, water can damage the board material and reduce its strength and adversely affect other mechanical properties. Sheeting intended for use during construction needs to be brought on site shortly before use, stored out of the weather, fitted and sealed. When used as soffit sheeting, both sides of the cellulose fibre-cement sheets should be sealed or a water proof lining should be used behind (above) the cement sheets.

The size of the fibres in the sheets and the imperfections in the surface limit the use of cellulose fibre-cement products. The boards can require various surface treatments such as sanding, priming and the application of multiple layers of paint in order to provide a suitable surface presentation. For example, when pre-sanded fibre cement sheets are used as an external wall lining, the joins between sheets are taped over and set with a thin layer of textured coating. Alternatively, a polymer render may be used over the entire wall. The wall surface may then primed with a trowel-on texture finish such as Dulux Professional UltraSmooth Cement Sheet Surfacer. The textured finish helps to reduce noticeable surface imperfections and provides a smooth, water resistant preparation coat. The treated sheets are then painted with two or more coats of acrylic paint finish. Even with the preparation treatment, it is recommended that the sheets are only painted with low sheen acrylic paints and used in selected lighting environments in order to minimise noticeable surface imperfections.

The existing fibre-cement soffit sheets and liner boards are not suitable for use under the eaves of residential housing due to the additional surface preparation requirements, on site labour costs and the problems with the final painted appearance of the sheeting.

There can also be difficulties in fastening the product to the rafter tails and soffits. Fibre- cement products are not as compressible as the earlier wooden liner boards or metal soffit sheets. The products are more brittle and can crack or break when nailed. When nails are used, typically specialised fibre cement nails are required and the nails should not be used near the edges or corner of the sheets. Instead, the boards may be pre-drilled and screws may be used to fasten the boards. Adhesive formulations may also be used, such Liquid Nails®. However, contractors working over head do not favour such systems. These difficulties have generally limited the use of fibre cement soffit sheets to business properties, industrial properties, garages and sheds where the final appearance is not as important.

Summary of the Invention The present invention is predicated on the finding that it is possible to successfully powder coat fibre-cement sheets to provide a powder coated product suitable for use under the eaves of residential housing. This is surprising given the water content of fibre-cement sheets which could preclude the use of powder coating techniques. The use of powder coated fibre- cement sheets, preferably cellulose fibre cement sheets, as liner boards for eaves or soffit sheets offer significant advantages over the traditional products.

In an embodiment of the invention there is provided powder coated fibre-cement soffit sheets and eaves liner boards. In an embodiment of the invention there is provided the use of powder coated fibre-cement sheets as soffit sheets and eaves liner boards.

In an embodiment of the invention there is provided a process for manufacturing powder coated fibre-cement soffit sheets or eaves liner boards whereby a low temperature and / or UV curing powder coating composition is applied to fibre-cement sheets and cured to provide a powder coating on said sheets or boards.

Preferably the fibre-cement substrate is a cellulose fibre-cement substrate.

Preferably the powder coating composition used to coat the cement substrate is a low temperature curing and / or UV curing composition.

In another embodiment of the invention there is provided a powder coating composition which comprises a polytetrafluoroethylene (PTFE) polymer. The composition may comprise from 0.2 to 4%, preferably 0.3 to 3%, more preferably 0.5 to 2 % and most preferably approximately 1% by weight of PTFE. It is speculated that the use of texture agents such as PTFE polymer provides a better coating with less noticeable defects on the fibre-cement substrate. In a preferred embodiment the PTFE polymer is Dyneon TF 1641.

Preferably the powder coating composition comprises PTFE together with a degassing agent. Preferably the degassing agent comprises an amide modified phenolated urea surfactant such as Powdermate 542DG. Other degassing agents such as Benzoin may be used in combination with the Powdermate product.

In another embodiment of the invention there is provided a method of powder coating substrates which have a significant moisture content by the inclusion of a suitable amount of a PTFE polymer within the powder coating composition. Preferably the powder coat composition also includes an amide modified phenolated urea surfactant such as Powdermate 542DG and optionally additional degassing agents. Preferably the above powder coating compositions are used in the manufacture of the soffit sheets and eaves liner boards contains a texture agent.

In another embodiment of the invention there is provided a powder coating composition for use on fibre-cement sheets comprising from 0.3 to 3% by weight of a PTFE polymer and from 0.5 to 4% by weight of an amide modified phenolated urea surfactant such as Powdermate 542DG.

In another embodiment of the invention there is provided a powder coating composition for use on fibre-cement sheets comprising from 0.3 to 3% by weight of a PTFE, such as Dyneon TF 1641, or a micronised wax, such as Ceraflour 969, together with from 0.2 to 4% by weight of a degassing agent such as Powdermate 542DG and / or Benzoin.

Description of the Invention The term "powder coating" is often used with different meanings in the art. It may refer to the act of forming a powder coated substrate, the powder composition or formulation used in the formation of the coating or to the formed and cured coating layer. To avoid any confusion the terms "powder particles", "powder coating composition" and "powder coating formulation" refer to the powdered composition and "powder coating" refers to the application and curing process or the formed and cured layer on the substrate.

The fibre-cement sheets used to provide the soffit sheets and eaves liner boards may be manufactured with a range of fibres. Due to health risks, in most countries fibre cement sheets are manufactured without asbestos and instead use cellulose fibre such as unbleached plantation wood pulp. Other fibres known to be used in fibre cement products include steel, glass, polypropylene, acrylic fibres, akwara, alumina, carbon, coconut, kevlar, nylon, perlon, polyethylene, rock wool and sisal.

Each sheet or eave liner board may have a uncoated thickness from 3 mm up to 10 mm, with 3 mm to 8 mm preferred. Thickness of 4, 4.5, 5, 6, 7 or 7.5mm can be used. As the fitted sheeting will not ordinarily be exposed to knocks and blows then it is not necessary to use the thicker grades, and it is preferred to use a thickness of 3 mm, 4.5 mm or 6 mm. The liner boards may be thicker than the soffit sheets to give the boards greater strength since they are narrower in width. If the liner boards are too thin the boards may bow and snap during handling and fitting.

The products can be provided in a range of widths depending on the manufacturing process used and any cutting steps. The width can be from 90 mm to 2000 mm or larger. Preferred widths for soffit sheeting can be from 400 mm to 1500 mm, with 450, 600, 750, 900, 1000, 1100 and 1200 mm being typical widths. The width of eave liner boards may be from 50 to 150 mm, with 50, 80 and 100 mm boards preferred.

The length of each sheet or board varies depending on the manufacturing process and any subsequent cutting steps. The sheets and boards may be provided as short as 1000mm or as long as 4500 mm (or longer). Typical lengths include 1200, 1800, 2100, 2400, 2700, 3000, 3300 and 4200 mm.

The fibre cement sheet material can be formed using conventional techniques well known in the industry. For example the sheets may be manufactured using a Hatschek machine. Sieve rollers rotate in a fibre cement slurry mix and a layer of fibre cement builds up on the roller. Suction boxes are used to remove excess water. Once the layer builds up to the desired thickness the fibre-cement is cut from the roller and laid out to form a sheet. These green (uncured) sheets may then be stacked between steel sheets before curing, typically by steam curing techniques.

A general purpose fibre-cement sheet may be formed from a mixture of cellulose, water, fine silica and Portland cement. High-pressure steam autoclaves may be used to cure the green sheets in order to create a durable, dimensionally stable sheet product which, when sanded, can be suitable for use as liner sheeting for walls, ceilings and soffits.

The sheets can be formed without the use of autoclaves and without pressing the sheet. The sheets can be air dried, and this may be preferred for selected types of fibres, particularly man-made fibres instead of cellulose fibres. The steam autoclave subjects the sheet to temperatures in the order of 180-190⁰C for up to 12 hours at around 12 bar. Manmade fibres such as poly vinyl alcohol fibres are generally not suitable for use at these temperatures alone or in a blend with cellulose fibres. Air curing can take two to four weeks.

Calcium silicate boards can also be used instead of general purpose fibre cement sheeting. Such boards are fire resistant and provide thermal insulation but more importantly are low in weight. The boards are formed from cellulose fibres, lime, silica, water and other additives. They may be formed using a cylinder sheeting machine such as a Hatschek machine.

The autoclaved aerated concrete could also be used as an alternative substrate in place of fibre-cement sheets. Whilst the aerated concrete is typically provided as blocks, it can also be provided in thin panels which can be used as a light weight alternative to cement sheets.

Aerated concrete is formed from cement, finely ground silica sand or pulverized fly ash, quicklime, water and an aeration agent. It does not include fibres. It has good insulation properties but has an open textured surface.

Aerated concrete is formed by making a slurry of water, cement, fine sand / ash and quicklime. The aerating agent is then added to the slurry. The aerating agent is typically aluminium powder which reacts with the hydroxide in the lime to produce hydrogen gas. After the mass hardens, it is cut into thin panels which are then cured by steam autoclave.

At least one face, the surface intended to be visible, of the fibre cement sheets should be powder coated. This surface should be substantially flat and smooth. Traditional manufacturing processes used for making fibre-cement sheets for use in under the eaves and in soffits may provide a suitably flat and smooth surface for coating without significant sanding. However, when the surface is rougher or the sheets are to be used in critical light locations which will show up surface imperfections, it is preferred to pre-sand the fibre- cement sheets and / use a substantially thicker or textured powder coat layer in order to hide the more significant surface imperfections. More preferably a textured powder coating is applied to the fibre-cement soffit sheets or eaves liner boards, which is then sanded to provide a smooth surface finish.

The powder coating may be a primer coat which is subsequently coated with top powder layer or, may include pigments and provide a single layer primer / final coating. The powder coat layer can provide smooth surface finish on the fibre cement boards and sheets. The powder coating may significantly reduce the need to pre-sand uncoated sheeting and provide a good painted surface finish. It also provides a water-proof layer which should reduce water damage to the fibre cement and provide for higher gloss coatings.

In general it is difficult to successfully power coat materials which are temperature sensitive, such as cement sheeting. The high curing temperatures associated with the powder coating process (generally about 200⁰C or higher) result in distortion or internal splitting (checking) of the sheets and outgassing (causing surface defects - pin holing) from the release of moisture and other volatiles from the panels. A further problem typically associated with the powder coating of composite materials is the low adhesion of the powder to the sheeting. Poor coverage is often a problem at the edges of the sheets.

When powder coating fibre cement sheets, it may be helpful to improve charge retention by pre-heating the substrate sheets to a temperature over 8O⁰C. This increases the amount of water at or near the surface of the sheets and permits a charge to be more easily held. Other techniques that may be used include applying a brief burst of water or steam to the sheets or applying a conductive coating to the substrate. The latter approach has some disadvantages, not the least being requiring an additional step and the waste, solvents and drying time associated therewith.

Another method for improving charge retention is to incorporate sufficient amounts of electrically conductive materials within the fibre-cement composite. The inclusion of metal fibres, the use of metal powders, inorganic salts such as sodium chloride, carbon black and other conductive materials as additives to the composite may significantly enhance charge retention. The powder coating composition may be applied to the fibre-cement boards and sheets and a powder coating formed by using techniques adapted from those used to coat reconstituted cellulose containing substrates, such as particle board, as known to the art. Fibre cement sheets may be damaged by high temperatures or by the application of moderate heat over a prolonged period. Furthermore, the coatings applied thereto and layer formed thereon can be damaged due to vapour emissions from the substrate. Thus, it is preferred to apply the powder coating composition using electrostatic techniques and in a way which minimises unnecessary heating of the substrate sheets during the curing of the coating. Preferred methods involve the use of UV curable powder coating compositions, powder coating composition which cure at low temperatures, such as those including low temperature curing agents, or a combination of both UV and low temperature curing powder coating compositions.

The powder coating process may also include other techniques used to improve the adherence of the powder coating to the substrate such as pre-heating the substrate for, by example 5 minutes at 70⁰C, or the application of a burst of steam shortly before coating. Low temperature curing techniques can be enhanced by the use of localised heating with IR lamps which reduce the exposure of the substrate to heat. Hg containing lamps have been found to be effective for UV curing.

The powder coating composition utilised may be any suitable commercially available powder coating composition. Typically, the powder coating composition will be based on polyester, epoxy, hybrid blends of polyester / epoxy, polyurethane and other suitable resins. Preferably, it will be UV curable and / or low temperature curable. Polyester resin systems are preferred for external use as epoxy resins can exhibit significant colour and structure degradation with long term exposure to sunlight.

The coating compositions are generally prepared by adding the required amounts of the raw materials into a premixer in which the ingredients are mechanically mixed, usually with a metal blade, to form a homogeneous mixture. This premix material passes through an extrading process. In this process the mixture is processed under heat (usually between 8O⁰C and 14O⁰C) and compounded using mechanical shear. This causes the powder coating composition to melt and act like a semi-liquid, and allows the ingredients to be intimately mixed into the powder coating composition. After leaving the extruder the material is cooled, generally on a chiller belt. The cooled mixture is then milled (ground) to the required particle size distribution for good application. A standard particle size distribution ranges from 2 to 200 microns, preferably 10 to 150 microns and typically around a medium size of 50 - 60 microns.

The coating powder is typically applied to achieve a cured thickness of 0.04 to 0.6 mm, and preferably less than 0.1 mm. The powder is typically applied at a thickness of from 0.08 mm to 0.13 mm. The substrate could be coated with multiple layers to increase the thickness of the coating.

Vertical (hanging substrate) or horizontal coating systems could be used in the coating process. Each system has advantages. Horizontal powder coating systems, such as that described in US 2003/0211252, may be of particular use with longer lengths of substrate than could be reasonably attached to a hanging conveying system and should permit the powder coating of the main contact face and edge faces. Hanging systems allow the substrate to be entirely coated in a single pass with multiple electrostatic guns that apply the powder to all sides. Alternatively, electrostatic guns could be used to spray the powder on at least one face of a suspended substrate. Horizontal systems can be used to provide a wholly coated substrate in sequential powder coating steps. Horizontal systems can also allow the use of alternative powder delivery techniques such as fluidised beds or allowing powder to fall onto the substrate, by using for example a vibratory hopper.

The resin may contain colour pigments, extender pigments, cross-linkers and other additives. Examples of pigments and fillers include metal oxides, such as titanium oxide, iron oxide, zinc oxide and the like, metal hydroxides, metal powders, sulphides, sulphates, carbonates, silicates such as aluminium silicate, carbon black, talc, kaolins, barytes, iron blues, lead blues, organic reds, organic maroons and the like. A slip-enhancing additive may be included to improve coating wear characteristics such as that described in US 5,925,698.

Powder coating compositions may contain other coating modifiers such as polytetrafluoroethylene modified waxes, polyethylene waxes, polypropylene waxes, polyamide waxes, organosilicones and blends of the above.

The powder coating composition can be applied to any substrate by any suitable electrostatic technique. The two major techniques used are the corona electrostatic technique and the triboelectrostatic technique. According to the corona electrostatic technique the powder particles are given an electric charge as they come out of the end of a powder coating corona gun by electrodes located at the end of the gun tube. The electrodes are powered by a power- pack which can generate up to 100,000 V (100 KV). The usual working range for voltage is 50 to 100 KV. The powder is sprayed (powder is carried in a stream of air) at the earthed composite panel. The charge on the powder particles allows the powder particles to adhere to the substrate. After the powder coating is sprayed, a baking process is required to melt and chemically react and cross-link (creating a thermoset paint finish) the powder coating resin and the cross-linker.

The triboelectrostatic technique involves a tribogun which also works by charging the powder particles towards an earthed panel. The charge in this case is not generated by a power pack. The tribogun is generally a long polytetrafluoroethylene (PTFE) tube. Friction is generated between the powder coating composition and the PTFE tube and a charge on the powder is generated by electron removal.

Other powder application techniques are known and could also be used to apply the powder coating composition to the substrate. A technique that could be used is described in US Patent No. 6,342,273 (Handels, et al). The technique involves first charging the powder particles by friction or induction in the presence of carrier particles, feeding the charged powder and carrier particles to a transporter, transferring the charged powder particles from the transporter onto a transfer medium and then applying the powder particles from the transfer medium to the substrate.

It should be understood from the above that the powder coating process provides an environmentally friendly method of applying and forming a coating as a solvent is not required and the overspray particles that are not bonded to a substrate can be collected and re¬ used in the next powder coating application.

A broad range of powder coating compositions and application methods could be used. A technique for powder coating the substrate of the invention involves the use of UV curable powder coating compositions. With such coating compositions, the powder is applied to the substrate and heated to and above the melting point of the powder coating composition. The temperature achieved in the melting phase is usually between 9O⁰C and 16O⁰C. The melting phase is conducted by either infrared (IR) heating oven or convection gas or electric heating oven, or a combination of the two systems. After melting and flow out of the powder stage the coated panel is then passed under a UV cure oven. At this stage the coating is irradiated with UV light. The UV light phase is generated by either a mercury lamp or a gallium doped mercury lamp with wavelengths of between 205 and 405 nm.

Photo-initiators suitable for inclusion in UV powders include aromatic carbonyl compounds, such as benzophenone and alkylated or halogenated derivatives, anthraquinone and its derivatives, thioxanthone and its derivatives, benzoin ethers, aromatic or non-aromatic alphadiones, benzol dialkyl acetals, acetophenone derivatives and phosphine oxides.

The UV cure powder coating compositions can be applied to the substrate using similar techniques to standard coatings which require baking.

Low bake powder coatings are designed to cure at temperatures between 9O⁰C (or less) and 16O⁰C for between 10 and 40 minutes total oven time (in a conventional gas or electric fired oven). IR cure of low bake powder will be much faster (from 30 seconds to 5 minutes). Low bake powder coatings can be different chemistries. Epoxy and acrylic resins are commonly used. Combination UV and low temperature cure compositions use a combination of the two techniques described above. US Patent No. 5,922,473 and 6,005,017 includes a more detailed description of such techniques.

Some examples of some typical powder coating formulations are provided below.

Typical UV Cure Powder Coating Compositions UV Cure Resin 50-95% UV Initiator 1-3% Pigments 1-30% depending on colour Additives 0-5% as required

The UV cured resin will generally have the following properties:

Glass transition Temperature of: 50 +/-1O⁰C Unsaturated equivalent weight 1300g/eq +/-200 Cone/Plate Viscosity (175⁰C) 6000mPa.s +/-1000

The UV curing resin is generally an amorphous resin that can be cross-linked by a free radical polymerisation mechanism. The coating resin may contain unsaturated functional groups including methacrylic and acrylic unsaturated groups.

The UV initiators are added to start the free radical polymerisation mechanism, upon absorption of the UV irradiation energy. These can include but are not exclusive to α- hydroxyketone types such as l-(4-(2-Hydroxyethoxy)-phenyl)-2-hydroxy-2-methyl-l- propane-1-one and BAPO bis cyclophoshinoxide types such as Bis (2,4,6-trimethylbenzoyl)- phenylphosphineoxide). Other additives can be added as required.

Typical Low Temperature Cure Epoxy Powder Coating; Composition Pigments 0-40% Epoxy resin 40-90% Cross-Linker 0-16% Additives 0-10%

The epoxy resin generally has the following properties:

Epoxide equivalent weight: 400-900 Viscosity: 500-5000 Centistokes at 15O⁰C Softening point: 70-11O⁰C

Various cross-linkers can be used including but not limited to dicyandiamide or adducts of 2- methyl imidazole

Typical Low Temperature Cure Polyester Powder Coating Composition Polyester 50-95% Cross-linker 1-16% Pigments 1-40% depending on colour Additives 0-5% as required

The polyester resin usually has the following properties:

Acid Value (or hydroxy value) 20-80 Viscosity 200-700 dPa.s (at 165⁰C) Glass Transition Temperature 50-70⁰C

A flow additive may be present in any of the compositions in an amount from 0 - 3%, typically 1%.

The above formulations may contain between 0.3 to 3%, more preferably 0.5 to 2 % and most preferably approximately 1% of a polytetrafluoroethylene (PTFE) product, such as that marketed as Dyneon TF 1641 or Ceraflour 969, the latter providing a micronised wax containing a combination of PTFE and polyethylene. PTFE polymers include Teflon, an additive sometimes included in very small amounts in powder coating formulations as a slip additive. Ceraflour 969 is a wax additive for providing scratch resistant flat surfaces.

It is speculated that it is the use of PTFE polymers in an amount from 0.2 to 4% which provides a better coating with less noticeable defects on fibre-cement substrates and may also be useful on other types of substrates. It may be a useful additive for inclusion in powder coating compositions intended for use on moisture containing substrates, including substrates having a moisture content equal to or greater than 6%. It may be useful on manufactured or engineered woods including reconstituted wood substrates such as hardboard, medium density fibreboard (MDF), wafer board, flake board, chip board and particle board. It may also be a useful additive for powder coating compositions intended for plastics, glass, paper and metallic substrates.

The powder coating compositions should contain degassing agent and it is thought the combination of PTFE polymer together with the degassing agent provides a useful additive for use when powder coating moisture containing substrates. The degassing agent may be present in an amount of from 0.2 to 4% by weight. The preferred degassing agents are amide modified phenolated urea surfactants, such as Powdermate 542DG. Other degassing agents could also be used alone or in combination with amide modified phenolated urea surfactants. Benzoin products can also provide good results with fibre-cement substrates. The Benzoin product is preferably present in an amount of from 0.3 to 1%, more preferably about 0.5%. The Powdermate product is preferably present in an amount of from 1 to 4%, more preferably about 2% by weight.

A particularly preferred powder coating composition includes both PTFE in an amount by weight from 0.3 to 3% and a degassing agent selected from amide modified phenolated urea surfactants, such as powdermate 542DG, in an amount by weight from 1 to 4%. This mixture may be provided as an additive for use with typical powder coating compositions. The other components of the powder coating composition may be selected from a broad range of components such UV or temperature curing polymer(s), pigments and initiators or cross-linkers. The present invention will now be described with reference to the following non-limiting examples.

Examples Unless otherwise indicated the mixtures were prepared by combining the ingredients (resins, initiators, colour pigments, extenders, flow additives and other minor additives). The mixture was then agitated and then heated and extruded at 100⁰C to provide a homogenous sheet. The sheet was cooled, granulated and then milled and sieved to provide particles having a particle size less than 125 micrometers (average particle size of 40 microns) to provide the powder coating composition. All amounts are parts by weight.

The powder coating compositions were applied electrostatically to the substrate material and cured. UV curing used Hg (Mercury) and Hg/Ga (Gallium Doped Mercury) UV lamps. Heat curing involved the use of an IR (Infra Red) oven.

The substrate material was panels of normal fibre cement sheeting.

Table 1. - UV Cure Formulations

Coating formulation 1 was applied to and cured on a panel of normal fibre cement sheet which had been preheated for 5 minutes at 7O⁰C. The applied powder composition was heated by IR for 2 minutes at a IR setting of 200⁰C and cured with 3 passes of UV.

The coated fibre cement sheet panel was examined and found to provide a hard (no marring) and a textured and dimpled surface. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted.

Coating formulation 2 was applied to a panel of fibre cement sheet that had been preheated for 5 minutes at 7O⁰C. The composition was IR treated for 3 minutes at an IR setting of 200⁰C and subjected to 3 UV passes.

Coating formulation 3 was applied to fibre cement sheet which had been preheated for 5 minutes at 7O⁰C. The composition was IR heated for 1 minute at an IR setting of 200⁰C and by two passes through UV. The coated fibre cement sheet panel was examined and found to provide a hard (no marring) and a smooth surface. The coated surface of the panel was water tested with steam for 3 minutes and examined. No water damage was noted. Table 2 - Low Temperature Cure Formulations

Panels of fibre-cement sheeting were preheated at various periods ranging from no preheating, to preheating at 2 minutes at 85 ⁰C to 5 minutes at 16O⁰C before the application of the powder coating formulation. The panels were then cured for 20 minutes at 16O⁰C.

The panels of coated fibre cement sheet were examined and found to provide a hard (no marring) and smooth surface. The coated surface of the panels were water tested with steam for 3 minutes and examined. No water damage was noted.

The coated substrates were examined for surface defects and structure and were found to provide satisfactory results.

Whilst the above examples are single coat formulations, it is envisaged that significant benefits may be provided by using a textured primer coat, particularly when used with high gloss coatings, as typically provided when powder coating.

The primer layer would include larger amounts of texture additive such as Bentone, a fine form of organically modified hectorite clay, in addition to a PTFE polymer such as Dyneon TF 1641 or Ceraflour 969.

A typical low temperature / UV cure primer coating has the following formulation.

Table 3 - Prophetic Textured UV Cure Primer Formulations

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form or suggestion that that prior art forms part of the common general knowledge in Australia.

It would be appreciated by a person skilled in the art that variations and/or modifications may be made to the invention as described without departing from the spirit or scope of the invention as broadly described. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A powder coating composition which comprises a polytetrafluoroethylene (PTFE) polymer.

2. The composition according to claim 1 wherein PTFE comprises from 0.2 to 4% by weight of the composition.

3. The composition according to claim 2 wherein PTFE comprises from 0.3 to 3% by weight of the composition.

4. The composition according to claim 3 wherein PTFE comprises from 0.5 to 2% by weight of the composition.

5. The composition according to claim 4 wherein PTFE comprises approximately 1% by weight of the composition.

6. The composition according to any one of claims 1 to 5 wherein Dyneon TF 1641 or Ceraflour 969 provides the PTFE polymer.

7. The composition according to any one of claims 1 to 6 wherein the composition also includes a degassing agent.

8. The composition according to claim 7 wherein the degassing agent comprises an amide modified phenolated urea surfactant.

9. The composition according to claim 8 wherein the degassing agent comprises from 0.5 to 4% by weight of the composition.

10. The composition according to claim 8 or 9 wherein the degassing agent comprises Powdermate 542DG and / or Benzoin.

11. The composition according to any one of claims 1 to 10 when used to powder coat fibre-cement sheets.

12. The composition according to any one of claims 1 to 10 when used to powder coat fibre-cement sheets for use as soffit sheets or eaves liner boards.

13. The use of the composition according to any one of claims 1 to 10 in the manufacture of powder coated fibre-cement soffit sheets or eaves liner boards.

14. A powder coated fibre-cement soffit sheet or eaves liner board.

15. The use of powder coated fibre-cement as soffit sheets or eaves liner boards.

16. A process for manufacturing powder coated fibre-cement soffit sheets or eaves liner boards whereby a low temperature and / or UV curing powder coating composition is applied to fibre-cement sheets and cured.

17. The soffit sheet or eaves liner board of claim 14, the use of claim 15 or process of claim 16 wherein the fibre-cement is cellulose fibre cement.

18. The soffit sheet or eaves liner board of claim 14, the use of claim 15 or process of claim 16 wherein the powder coating contains a texture agent.

19. The soffit sheet or eaves liner board of claim 14, the use of claim 15 or process of claim 16 wherein the powder coat is formed from a powder coating composition according to any one of claims 1 to 10.